a) Field of the Invention
The present invention relates to a bearing unit, in which a turning shaft (shaft) inserted into an oil-impregnated material part is supported so as to be able to turn, and in particular, relates to a bearing unit that can prevent a lubricant existing between an inner circumferential surface of the oil-impregnated material part and the turning shaft from leaking out.
b) Description of the Prior Art
In the past, there have been fluid dynamic bearing units used for supporting polygon mirrors, optical discs, and so on that turn. In such a fluid dynamic bearing unit, a fluid dynamic pressure surface of the turning shaft and a fluid dynamic pressure surface of the oil-impregnated material part, which are both ring-shaped and have a clearance between the two in a radial direction, are placed so as to face each other. Then, the lubricant fluid such as oil and so on, injected into the clearance space, is pressurized at the time when the turning shaft turns. This is done by pumping operation as a means of generating fluid dynamic pressure, which is set up for either or both the turning shaft and the oil-impregnated material part. As a result, the turning shaft is supported so as to be able to turn relatively to the oil-impregnated material part, by the fluid dynamic pressure of the lubricant fluid.
Among those fluid dynamic bearing units, some are provided with grooves, such as herringbone-shaped, spiral-shaped, and so on, in order to generate fluid dynamic pressure as a means of generating fluid dynamic pressure. Furthermore, there is also a step-groove fluid dynamic bearing unit (Rayleigh step fluid dynamic bearing unit), having grooves (concave sections) on an inner circumferential wall surface that are deeper in depth and provided with a larger cross section than the grooves, such as herringbone-shaped, spiral-shaped, and so on, in order to generate fluid dynamic pressure as a means of generating fluid dynamic pressure. (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 3-107612 (FIG. 1).
FIG. 9 is a widthwise cross sectional view to show a mechanical structure of a conventional step-groove fluid dynamic bearing unit 100.
In FIG. 9, the conventional step-groove fluid dynamic bearing unit 100 is provided with, for example, fluid dynamic pressure generating surfaces 102a at 3 locations intermittently; each of which is equipped with a convex surface protruding in a radial direction with a stepped shape, on an inner circumferential wall surface of an oil-impregnated material part 102 surrounding a turning shaft 101 in a circumferential direction. When a lubricant fluid 103, such as oil and so on, (flowing in a CW direction in FIG. 9) is pressurized while being squeezed at a narrow space formed between each fluid dynamic pressure generating surface 102a and an outer circumferential surface of the turning shaft 101, fluid dynamic pressure is generated as required.
Furthermore, concave sections 102b are provided at 3 locations intermittently, while adjoining to each edge section of the fluid dynamic pressure generating surface 102a and forming grooves with a stepped shape in a radial direction. The concave sections 102b are placed for the purpose of canceling negative pressure that is caused together with the fluid dynamic pressure generated at the fluid dynamic pressure generating surface 102a. Each concave section 102b has a groove of a few microns to 0.1 mm approximately in depth in its inner radius direction, and it can be formed by making a copy of a shape of a forming core and a sizing core, having the same shape as required, on an internal surface of the bearing section material. Moreover, porous minute holes in the fluid dynamic pressure generating surface 102a are made to be fine and fewer, while those in the concave section 102b are made to be rough and more.
The step-groove fluid dynamic bearing unit 100 described above by referring to FIG. 9 is suitable for spindle motors free from any lateral load, especially for spindle motors of floppy disc drive units.